Rapidly increasing use of fossil fuels has led to an increase in demand for use of alternative or clean energy. In light of such trends, generation and storage of electricity using electrochemical reaction are a very active area of research.
In recent years, representative examples of electrochemical devices using electrochemical energy are secondary batteries, and application thereof continues to expand.
Recently, technological development and increased demand associated with portable equipment such as portable computers, cellular phones and cameras have brought about an increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge have been actively researched and are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles, hybrid electric vehicles or the like as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles. These electric vehicles and hybrid electric vehicles generally use nickel-metal hydride secondary batteries as power sources. However, a great deal of study associated with lithium secondary batteries with high energy density and discharge voltage is currently underway and some are commercially available.
Conventional typical lithium secondary batteries use graphite as an anode active material. Lithium ions of a cathode are repeatedly intercalated into and de-intercalated from the anode to realize charge and discharge. The theoretical capacity of a battery varies depending upon the type of the electrode active material. Deterioration in charge and discharge capacity of a battery generally occurs in the course of the cycle life of the battery.
The primary reason behind such phenomenon is that separation between an electrode active material, or separation between the electrode active material and a collector due to volume variation in the electrode as batteries in the course of charging and discharging results in insufficient realization of function of the active material. In addition, in the process of intercalation and de-intercalation, lithium ions intercalated into the anode cannot be sufficiently de-intercalated and active sites of the anode are thus decreased. For this reason, charge/discharge capacity and lifespan of batteries may decrease as the batteries are cycled.
In particular, in order to improve discharge capacity, in the case where natural graphite having a theoretical discharge capacity of 372 mAh/g is used in combination with a material such as silicon, tin or silicon-tin alloys having high discharge capacity, volume expansion of the material considerably increases, in the course of charging and discharging, thus causing isolation of the anode material from the electrode material. As a result, battery capacity disadvantageously rapidly decreases over repeated cycles.
Accordingly, there is an increasing demand in the art for binder and electrode materials which can prevent separation between the electrode active material, or between the electrode active material and the collector upon fabrication of electrodes via strong adhesion and can control volume expansion of the electrode active material upon repeated charge/discharge via strong physical properties, thus improving battery performance.
Polyvinylidene difluoride (PVdF), which is generally used as a binder of cathodes and anodes, is a polymer resin dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP). Although PVdF was not developed as an adhesive, it is generally used as a binder of electrode active materials, since it exhibits superior miscibility with a graphite material and realizes formation of an electrode plate with superior adhesion strength when added in about an amount of 8 to 10% of the graphite.
However, since PVdF covers an active material in a state in which polymer fibers are packed, the electrode active material deteriorates inherent battery performance in terms of capacity and efficiency. In addition, in the case where a material having a large specific surface area like natural graphite or metallic active materials and exhibiting high expansion and contraction ratio upon charging and discharging is used as an electrode active material, the bond may be readily cleaved or cycle properties may be deteriorated due to insufficient flexibility of PVdF. Furthermore, PVdF absorbs a carbonate electrolyte and then swells, thus causing deterioration in power capacity in the course of cycles.
Another binder used for lithium secondary batteries as an aqueous binder is rubber-based latex such as styrene-butadiene rubber (SBR). SBR is environmentally friendly, reduces the amount of binder used and improves the capacity of secondary batteries and initial charge/discharge efficiency. However, in this case, adhesion persistency is improved due to elasticity of the rubber, but adhesion strength is not greatly increased. Accordingly, SBR exhibits great volume expansion when charged/discharged and thus entails restriction in use such as inapplicability to active materials with high capacity which require electrodes with high adhesion strength.
Accordingly, there is an increasing need for development of binders which improve cycle properties of batteries and structural stability of electrodes and exhibit superior adhesion strength.